The Mystery of Bright Nights – Solved

When we lived in the crowded south east of England, in Brighton, the night skies were the colour of iron-rich mud, a nasty orange-brown. Now, in remote north Devon, we’re rediscovering night’s extraordinary natural variety. As it turns out, the night isn’t always dark. In fact every night is unique, and the oddest nights of all are ‘Bright Nights’.

It’s tricky to describe a Bright Night, a night that’s almost as bright as day. But here goes. The first time we experienced a Bright Night there was no moon, but deep shadows sharply defined the landscape anyway. The fields, copses and lanes around our house became silvered with a strange, pale, milky light. It was uniformly cloudy and the light seemed to radiate from the earth rather than the sky. Once we were up on the moor we could literally see for miles, with distant Exmoor defined as clear as day. It really was freaky.

Since then I’ve kept a close eye on our nights, discovering we also get moonlit Bright Nights. On ordinary moonlit nights, despite a powerfully bright moon, the landscape remains impenetrably dark, too dark under the trees to see beyond a few metres and exactly what you’d expect from a rural setting with no light pollution. On moonlit Bright Nights the moon is no brighter than normal but we get this remarkable light that suffuses the landscape, clear and strong and non-directional. You can see just as clearly under the trees as you can in the middle of a field.

What causes Bright Nights?

Explore the historical record and you see all sorts of reports about Bright Nights, when weird luminous light brightened the night time landscape in every direction like sunlight. The earliest account comes from the Roman commander Pliny the Elder, dating back to around AD 77. It has been a mystery for a very long time.

There’s some early insight from way back in 1909, when a student in the Netherlands measured the total amount of starlight reaching the earth’s surface and found a discrepancy on Bright Nights. He wondered whether it was an atmospheric phenomenon and – poetically – called it ‘earthlight’. But that didn’t really explain it.

Luckily some clever folk have been on the case more recently. John Barentine, an astronomer at the International Dark-Sky Association, found a fascinating clue in 19th century papers talking about a peculiar ‘luminous smog’. Observers at the time described it as very different from an aurora and nothing like ‘zodiacal light’ which is a lot dimmer, caused by fine space dust reflecting sun rays from below the horizon.

In the late 1980s Gordon Shepherd of York University in Toronto landed on another clue. He built WINDII, a chunk of satellite tech designed to monitor air waves as they roll through earth’s atmosphere, and thanks to it he found these waves could pile up to create vast towers of pressurised air, which affected the quality of light. His studies also revealed how the sun’s UV radiation splits molecular oxygen into individual atoms, which join up again as soon as the sun sets. The small amount of light you get as a result is called ‘airglow’, something that varies dramatically from night to night, place to place. And that sounds an awful lot like a Bright Night.

Shepherd decided to find out whether air waves and airglow were connected. If his air waves led to a higher concentration of oxygen, the resulting intense glow could explain Bright Nights, and pin down why the light on a Bright Night comes from every direction. The pattern fit, and eventually, along with his colleague Young-Min Cho, he found Bright Nights can occur 7% of the time at any given place. Which means we get as many as 25 Bright Nights per year. And that chimes perfectly with our experiences in north Devon. What many thought was a rare occurrence is actually relatively common.

Our latest Bright Night happened last week, a moonlit one, and yet again I was staggered by its eerie beauty. These days runaway man-made light pollution means few of us get to see the phenomenon for ourselves. A whopping 99% of us in Europe sleep under an artificially lit sky whose subtleties are completely hidden from us. And that seems a shame when every night is unique, every night is different.

Thanks to New Scientist magazine for the science bits – the planet’s finest weekly read.

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